Current operating light water reactors (LWR) utilize fuel assemblies that have a square cross-sectional area in which the nuclear fuel rods are located. Light water reactor designs employ a square array for the layout for control rod drives and consequently the area allocated for fuel assemblies is square. The fuel rods are distributed in the available square area so that there will be an approximately uniform distribution of coolant/moderator area for each fuel rod. The approach has been to arrange the fuel rods within the available square area so that there was an equal number of rows and columns of fuel rods with a uniform center-to-center distance (i.e. pitch) between fuel rods. This arrangement is referred to as a square lattice, as lines drawn through adjacent fuel rod centers divide the area into a number of uniform squares. The reactor power and power distribution (axial, radial and local peaking) set the volumetric power density generated in the fuel rods. The minimum spacing between fuel rods to assure adequate cooling of adjacent fuel rod surfaces, which has been determined by heat transfer tests, must be provided with allowance for manufacturing tolerances and predicted fuel rod bowing during operation. For a uniform array of fuel rods, the required minimum rod-to-rod spacing limits the maximum allowable fuel rod diameter for that array. Uniform distribution of uranium fuel and coolant moderator (i.e. water) has been typically obtained by selecting an equal number of rows and columns of fuel rods in a square lattice array and positioning the center of the nuclear fuel rods at the corners of the squares. Thus, the number of rows of fuel rods equal the number of fuel rods in a row. The fuel rod array is sized to obtain sufficient heat transfer area for the volume of nuclear fuel in a fuel rod to enable the removal of the heat generated by the fuel within temperature limits of the materials used for the fuel rod.
Boiling water reactor (BWR) fuel assemblies typically have such a fuel rod array in which the fuel rods are arranged in rows with the same number of fuel rods in each row as there are rows in the array. In adjacent rows, fuel rods are located with their centers at the corners of squares. Such square rod arrays or lattices are commonly named according to the number of rows of rods and number of rods in a row such as 8.times.8, 9.times.9, 10.times.10, etc. Regardless of the number of rows of rods, each array is constrained to fit within a standard size fuel assembly channel.
The use of a square lattice whereby fuel rods are located with their centers at the corners of squares results in a larger flow area at the center of the square formed by four fuel rods than is necessary. This is an inefficient use of the cross-sectional area within a fuel assembly channel. It is desirable to reduce the fuel rod linear heat generation rate and the internal fuel rod temperature for a given fuel assembly power level by increasing the number of fuel rods. This is done, for example, by changing from a 10.times.10 fuel rod array to an 11.times.11 array. Since the fuel rod array is constrained to fit within the fixed dimensions of a standard fuel assembly channel and is required to have a certain minimum fuel rod surface to surface and fuel rod surface to channel wall surface spacing, increasing the number of rows of fuel rods and number of fuel rods in a row necessitates a decrease in the fuel rod diameter. The fuel rod diameter must be reduced to maintain surface to surface spacing since the fuel rod center to center distance is reduced. The spacing between rods to allow for adequate cooling and to accommodate fuel rod bow cannot be reduced in proportion to the rod-to-rod pitch. As the quantity of the fuel rods is increased in a square lattice, the increased number of fuel rods will not compensate for the required fuel rod diameter reduction with the result that the uranium loading in the fuel assembly is reduced in the finely divided array.
For example, a 10.times.10 square lattice array would have a rod pitch of approximately 0.51 inch and a minimum rod surface to rod surface space that would allow for manufacturing tolerances, and rod bow to maintain adequate cooling throughout the operating life. If such a space were 0.114 inch, then the maximum rod diameter could be 0.396 inch. If the square lattice array was more finely divided to an 11.times.11, then the rod pitch would be approximately 0.464 inch. The maximum rod diameter would be limited to 0.35 inch to maintain the required 0.114 inch space between rods. The amount of space for fuel is proportional to the number of rods and their cross sectional area. The relative fuel cross sectional area for the two arrays would be ##EQU1##
In BWR fuel assemblies, a number of fuel rod locations are reserved for use instead as water rods or a water channel to selectively increase neutron moderation for more efficient fuel utilization. If the square fuel rod array is more finely divided and if the number of reserved water rod locations remains constant, then the amount of moderating water within the water rods or water channel becomes smaller because of the smaller allowable diameter for both the fuel rods and water rods. If the number of reserved rod locations for water rods is increased as the array size is more finely divided, then the uranium loading for the fuel assembly is decreased even further. Thus, as the square fuel rod array is more finely divided and the number of water rods either increases or remains unchanged, inefficient fuel utilization as well as high fabrication costs result.
A triangular lattice array in which the centers of fuel rods are located at the vertices of a triangle is more desireable than the square lattice array in that it provides a more efficient arrangement of fuel rods while also maintaining required rod-to-rod spacing. For a specified fuel rod diameter and minimum rod-to-rod spacing, the triangular lattice allows a tighter packing of fuel rods within the given cross sectional area of the fuel assembly channel, resulting in a better allocation of area for flow of coolant water among fuel rods. The higher density of fuel rods will permit a higher loading of uranium, and better heat transfer characteristics as the coolant water is on the average in closer proximity to the fuel rod surfaces. In addition, more fuel rod heat transfer surface can be incorporated in a unit area than in a square lattice array of the same pitch, and greater flexibility for internal moderation using water rods and inner water channels can be obtained. Since the higher density of fuel rods permits a higher loading of uranium as the number of fuel rods in the assembly is increased, more fuel rod locations can be reserved for water rods or water channels without causing a decrease in the uranium loading in comparison to a square lattice array which will have fewer fuel rod positions.
A triangular lattice however cannot be made to fit into a square cross-sectional area by having an equal number of rows and columns of fuel rods.
It is an object of the invention to have a fuel rod arrangement in which a triangular lattice is utilized for fuel assemblies that are square.